1. Field of the Invention
The present invention is related to a power switch circuit capable of tracing a higher supply voltage, and more particularly, to a power switch circuit capable of tracing a higher supply voltage without a voltage drop, so as to prevent junction leakage as well as ensuring the drive capability of the power switch circuit.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a conventional voltage selecting circuit 10. The conventional voltage selecting circuit 10 comprises a first P-type Metal Oxide Semiconductor (PMOS) 11 and a second PMOS 12. The source end of the first PMOS 11 is coupled to a power voltage VDD and to a gate end of the second PMOS 12; the source end of the second PMOS 12 is coupled to a programming voltage VPP and to the gate end of the first PMOS 11. The body ends of the first PMOS 11 and the second PMOS 12 are coupled to the drain ends of the first PMOS 11 and the second PMOS 12, respectively. The drain end of the first PMOS 11 is coupled to the drain end of the second PMOS 12, for generating an output voltage VPPI. In applications such as memory devices where the conventional voltage selecting circuit 10 is utilized, the voltage level of the power voltage VDD is usually fixed at approximately 3.3 volts (V); the voltage level of the programming voltage VPP is usually approximately 0V, 3.3V, or 6.5V depending on the operation mode such as read mode or program mode.
Please refer to FIG. 1 and FIG. 2 together. FIG. 2 is a table illustrating the voltage level of the output voltage VPPI under different conditions of the power voltage VDD and the programming voltage VPP for the conventional voltage selecting circuit 10. When the voltage level of the power voltage VDD is 3.3V and the voltage level of the programming voltage VPP is 0V, the first transistor 11 is turned on, so the voltage level of the output voltage VPPI equals to that of the power voltage VDD, which is 3.3V. When the voltage level of the power voltage VDD is 3.3V and the voltage level of the programming voltage VPP is 6.5V, the second transistor 12 is turned on, so the voltage level of the output voltage VPPI equals to that of the programming voltage VPP, which is 6.5V. This way the voltage selecting circuit 10 is able to select/trace the higher voltage out of the power voltage VDD and the programming voltage VPP. However, the conventional voltage selecting circuit 10 is unable to accurately trace high in certain conditions. For instances, when the voltage levels of the power voltage VDD and the programming voltage VPP are both 3.3V, the first transistor 11 and the second transistor 12 are not turned on, so the current flows through the junction between the source end and the body end of each of the transistors 11 and 12. Therefore, the voltage level of the output voltage VPPI equals (3.3V−VTP), wherein VTP represents the threshold voltage of the first PMOS 11 and the second PMOS 12. The threshold voltage VTP is usually around 0.7V so the voltage level of the output voltage VPPI is only (3.3V−0.7V)=2.6V, when both of the power voltage VDD and the programming voltage VPP are 3.3V. Further, when the voltage level of the programming voltage VPP is floating, the programming voltage VPP can be any of the above voltage level but usually the programming voltage VPP tends to be low so it is assumed that the programming voltage VPP is lower than 3.3V. In other words, when the voltage level of the power voltage VDD is 3.3V and the voltage level of the programming voltage VPP is floating (i.e. usually lower than 3.3V), the first transistor 11 is turned on, so the voltage level of the output voltage VPPI equals to that of the power voltage VDD, which is 3.3V.
It is observed that the inability of the conventional voltage selecting circuit 10 to accurately trace high when the power voltage VDD approximately equals the programming voltage VPP (i.e. 3.3V) sometimes causes current leakage from the power voltage VDD to the ground terminal VSS due to the parasitic PNP junction occurring within the first transistor 11 and/or the second transistor 12. Also, another concern is that when the conventional voltage selecting circuit 10 is utilized as the power source to turn on a switch, the output voltage VPPI of (3.3V−VTP) may not be fully sufficient to turn on the switch and resulting in leakage current IOFF especially when the switch is also a MOS, causing inconvenience to the users.